Radiographic inspection of tube welds in panel walls

ABSTRACT

A method of inspecting tube welds in tubular membrane panel walls whereby a radiation source if placed intermediate adjacent tube welds with radiographic film positioned along the far wall of each adjacent tube thereby covering the far wall critical area of each tube weld. Simultaneously each radiographic film is exposed to an intermediate radiation source thereby creating an actual size image of each tube weld with both images totaling or equaling at least one full weld being radiographed per exposure.

FIELD OF THE INVENTION

This invention pertains to tubular membrane panel walls and moreparticularly to the radiographic inspection of the tube welds joiningtwo such panel walls together.

BACKGROUND OF THE INVENTION

It is now very common to construct steam generating facilities having afurnace constructed of prefabricated tubular membrane panel walls. Thesepanel walls serve as a heat-absorbing surface and also provide a gastight flow circuit for the generation of steam.

During the construction of the furnace, several panel wall units must bewelded together to create the elongated furnace enclosure. Normally thetubes of these panel wall units are welded end-to-end, but sometimes amid-region of a tube must be cut and re-welded for proper installation.No matter where the weld occurs, for safety's sake, the tube weld mustbe inspected to insure its compliance with a variety of guidelines,specifications, and codes. In particular, the critical portion of eachtube weld (where the tube and the adjacent membrane bar are joined) mustbe inspected to prevent any possible failure under pressure.

The preferred inspection technique is radiography which creates an imageof a number 1 tube welds simultaneously on radiographic film. Thisradiographic technique places the source of radiation some distance fromthe front of the panel wall with X-ray film being placed or taped on theother side of the panel wall. The radiation source is also verticallyoffset from the tube weld by about 15 to 20 degrees in order to separatethe tube weld image of the near wall from that of the far wall on theX-ray film. Additionally, this method requires no cutting or removal ofthe membrane between adjacent tubes. Unfortunately, this techniquecannot provide a clear image of the critical area of the tube weldbecause on the film the front critical area is superimposed over theback critical area, thereby distorting both images.

An improvement of this method is one where the source is not onlyvertically offset from the tube weld but is also offset to the right orleft of the tube weld. Accordingly, the radiographic film must also bemoved from its position adjacent to the back of the tube welds to aposition along the side of each tube weld so as to remain generallyperpendicular to the rays emanating from the offset source. This newposition of the film necessitates the removal of a segment of themembrane bar between adjacent tubes. The additional cost incurred toremove and replace this membrane bar segment is considered well spent ifthe critical area of the tube weld can be inspected. Unfortunately, thisis not the case because first, the offset of the source to the right orleft cannot be too great or else the adjacent tube will block orinterfere with the x-rays emanating from the source. Second, andprecisely because of this lack of clearance with adjacent tubes, thefront critical area of the tube weld will, on the film, still besuperimposed over that of the back critical area of the tube weld.Another drawback is the fact that with this method, two 'shots' or viewsof each tube weld are required (one for the left critical area and onefor the right) to obtain a full view of the tube weld; thus costs aresix fold. However, this second method does provide better coverage ofthe critical area than does the first method; unfortunately, it is stilla distorted view of the critical area and its coverage is still lessthan 100%.

It is thus an object of this invention to disclose an inspection systemwhereby 100% coverage of the tube weld is achieved including thecritical area. It is also an object of this invention to describe asystem whereby only one radiographic image need be taken to view a fulltube weld. A further object is to eliminate any distortion of the imageof the tube weld on the film. Another object is to provide an `actualsize` image of the tube weld to aid in its inspection and/orinvestigation. Still a further object is to reduce the exposure time andthe emissivity of the source required for each radiographic imagethereby increasing the margin of safety for the personnel using suchradioactive material. Yet another object of this invention is to reducethe time required to set up and complete each radiograph shot. These andother objects and advantages of the invention will become apparent uponfurther investigation.

SUMMARY OF THE INVENTION

This method of inspecting tube welds in tubular membrane panel wallscomprises the steps of installing a radiation source intermediate andadjacent to a pair of tube welds in a tubular membrane panel wall.Normally, to accomplish this, a portion of the membrane bar between theadjacent tube welds must first be removed. After such installation (orbefore if desired), a radiation receiver, such as x-ray film, is securedaround a portion of the perimeter of each tube weld. These radiationreceivers are bent or curved around their respective perimeter portionin order to conform to and cover at least 50% of each tube weld plus oneof the critical areas of each tube weld (a critical area being where atube and its adjacent membrane bar are joined). Once the above are inplace, the radiation receivers are exposed simultaneously to theradiation source thereby creating an image of the covered portion ofeach tube weld.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front pictorial view partially cutaway, of a conventionaljuncture between adjacent tubular membrane panel wall units after beingwelded together but before the gap between membrane bars are sealed. Thecritical area of the tube welds are circled.

FIG. 2 is a top view, partially cutaway and taken along lines 2--2 ofFIG. which illustrates one conventional manner of inspecting the tubeweld. In this illustration, the critical area of the tube welds arecircled.

FIG. 3 is a side view, taken along lines 3--3 of FIG. 2, whichillustrates the typical offset of the source from the horizontal.

FIG. 4 is a pictorial view of the radiographic image obtained by themanner depicted in FIG. 5. The image of the critical area is emphasized.

FIG. 5 is a top pictorial view partially cutaway, illustrating ingreater detail the conventional manner of inspecting tube welds as shownin FIG. 2.

FIG. 6 is a top view, with the membrane bar cut away, taken along lines6--6 of FIG. 7 and illustrating another conventional manner ofinspecting tube welds.

FIG. 7 is a front pictorial view, partially cut away, disclosing theremoved membrane required for the manner of inspection depicted in FIG.6.

FIG. 8 is a top view, with the membrane removed, taken along lines 8--8of FIG. 9 and illustrating applicant's manner of inspecting tube welds.

FIG. 9 is a front pictorial view, partially cut away, disclosing theremoved membrane required for applicant's manner of inspection depictedin FIG. 8.

FIG. 10 is a top pictorial view, partially cut away, illustrating ingreater detail applicant's manner of inspection depicted in FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

Referring initially to FIG. I, there is shown tubular membrane panelwall 10 comprised of flat membrane bars 12 welded between tubes 14.Adjacent units 16 and 18 would, as shown, be welded together to createone large tubular membrane panel wall 10. During the construction ofunits 16 and 18, membrane bars 12 are sized so that tubes 14 extendabout 1/4" beyond the end of bars 12. This 1/4" shortfall from adjacentends of units 16 and 18 in addition to the approximately 1/4" thicknessof tube weld 20 results in a total gap of about 3/4" between the ends ofadjacent membrane bars 12. During construction, this gap is closed bywelding to complete the wall seal in this region.

Referring now to FIGS. 2-5, there is shown the conventional double wallimaging method of inspecting tube welds 20. As shown, radioactive source22 is positioned some distance in front of to-be-inspected tube weld 20with source 22 generally consisting of iridium 192 isotope. Radiographicfilm 24, such as x-ray film, is placed on the opposite or back side oftube weld 20 and preferably tangent to tubes 14. Rays 26 radiating fromsource 22 pass through tube weld 20 and impact on film 24 therebycreating an image of tube weld 20 on film 24. To prevent superimposingthe image of near wall 28 of tube 14 over that of far wall 30 on film24, source 22 is vertically offset, either above or below, fromhorizontal plane 32 by about 15 degrees to 20 degrees (see FIG. 3). Theresulting double-wall elliptical image 33, as illustrated in FIG. 4,separates near wall 28 and far wall 30 into images 36 and 38respectively.

Of major importance is the ability to inspect the critical area 34 oftube weld 20. Critical area 34 is that portion of tube weld 20 that isadjacent to membrane bar 12. FIG. 5 illustrates the critical area 34 oftypical tube 14 and its respective image 35 on film 24. As can be seen,due to the orientation of each critical area 34 being generally parallelto rays 26, the area of image 35 of these critical areas 34 on film 24is relatively small (being located at the apex ends of elliptical image33 of FIG. 4). Additionally, and as further indicated in FIG. 4, image35 of critical area 34 has its near wall image 36 superimposed over itsfar wall image 38. Consequently, any information contained aboutcritical area 34 on film 24 is distorted. With this method only about65% of tube weld 20 can be imaged.

One typical method of resolving this distortion of critical area 34 isshown by the double-wall imaging radiographic technique illustrated inFIGS. 6 and 7. As indicated, source 22 is offset to the right and thento the left of tube 14 prior to the taking of each double wall image 33.

In this method, however, two separate images 33 are needed, one withrespect to each critical area 34 of each tube weld 20 thus necessitatingtwo separate set-ups for each tube. The positioning of source 22 wouldbe about 18 inches from film 24 and the two placements would form anangle of approximately 90 degrees. Unfortunately, however, thisside-to-side relocation of source 22 cannot be too great, otherwiseadjacent tubes 14 will interfere with rays 26 thereby distorting anycreated image 33. Additionally, a rather large segment of membrane bar12 (approximately 5 inches in length) will need to be removed (FIG. 7)so that film 24 can be properly positioned opposite its respectivesource 22. As indicated in FIG. 7, the removed segment need not becentered about tube weld 20.

A film size similar to that previously used (approximately 41/2") isrequired because of the magnification of tube weld image 33 on film 24due to outwardly radiating rays 26. The only portion of the image 33 oftube weld 20 that is actual size is that portion of film 24 tangent totube weld 20. The critical area 34 of tube weld 20 and its related image35 on film 24 is similar to that shown in FIG. 4 but taking into accountthe left and right shifting of source 22. With this method, however, thecritical area image 35 on film 24 is slightly moved away from theopposite apexes. Although this method is an improvement over the earliermethod, problems still remain and full coverage of tube weld 20 is stillnot attainable.

Applicant's contact method of radiography contemplated herein isillustrated in FIGS. 8-10. Under this method, source 22 is relocated toa position directly intermediate tube welds 20. Unlike conventionalwisdom, source 22 is not offset either vertically or horizontally withrespect to tube weld 20. Furthermore, film 24 is wrapped around and incontact with a portion (the far portion) of tube 14. In this fashion,and unlike the prior methods, no magnification of tube weld 20 occurs;instead, at least half of tube weld 20 is recorded on film 24 in 1actual size ` status, including the all-important critical area 34 offar wall 30. This method also enables, as shown in FIGS. 8 and 10, thecreation of two separate images of adjacent tube welds 20 from only asingle exposure. Furthermore, due to the `actual size` image created,the size of film 24 is significantly reduced, enabling film widths ofonly about 21/2 inches to be used as compared to film widths of up to 5inches or more required for the earlier methods due to the magnificationof tube weld 20 and the vertical offsetting of source 22.

As contemplated, with this method, the construction of units 16 and 18of tubular membrane panel wall 10 involves membrane bars 12 that areeither cut back an additional inch or so on each end or are manufacturedan inch or so shorter on each end. This would result in a gap betweenadjoining ends of membrane bars 12 of only about 21/2 inches.Consequently, a significantly smaller opening (less than half) iscreated which would need back field welding for closing thereby reducingcosts.

Film 24 would be curved or bent to fit around and contact the distant orfar critical area 34 of each adjacent tube weld 20. With thisarrangement and as indicated in the figures, film 24 would be spacedfrom source 22 no more than approximately 1 to 11/2 diameters of itsrespective tube weld 20. Any distance over one diameter is due to thewidth of membrane bar 12 and/or the thickness of tube weld 20. Normally,film 24 would surround at least half (FIG. 8) of each tube weld 20.Thus, for each simultaneous exposure, at least 360 degrees of tube weld20 is recorded resulting in 100% actual imaging of tube weld 20.

As better illustrated in FIG. 10, approximately half of each tube weld20 can be recorded as "actual size" because rays 26 pass directlythrough the tube weld 20 along far wall 30 before impacting on film 24.Because source 22 is in such close contact with the critical area regionadjacent near walls 28 and because it is positioned so closely betweenthe tube welds themselves, no image of this region will appear on film24. Instead, source 22 and this region will become verticallyundistinguishable and will be recorded on film 24 as though this wholearea adjacent tube welds 20 is the radiating source.

Once the images of adjacent tube welds 20 are made, source 22 is movedover one tube 14 and placed in the gap in the next membrane bar 12. Twosmall strips of film 24 are then placed around these two adjacent tubes14 to again cover at least the critical area 34 of the far tube weld 20of each tube 14 but at least 50% of each tube weld 20. Normally, film 24is held in place by tape or a hook and loop method of securement. Aftera typical exposure duration of about 30 seconds (as compared to three tosix minutes with other methods) the process is relocated and duplicatedat the next gap in membrane bar 12 until the entire panel wall 10 isinspected. Of course, when the end of a panel wall 10 is reached, theprocedure is modified so as to obtain a full view of this last tube weld20 in the manner described above. Normally, the weld type beinginspected is a butt weld of the V groove type. The material being weldedis oftentimes carbon steel but in some cases the boiler tubes may beoverlaid with stainless steel.

What is claimed as Invention is:
 1. A radiographic method of inspectingtube welds in tubular membrane panel walls comprising the steps of:(a)installing a radiation source intermediate an adjacent pair of tubewelds in a tubular membrane panel wall; (b) securing a radiationreceiver around a perimeter portion of each said tube weld, saidradiation receivers closely conforming to their respective perimeterportion and covering at least one critical area of its respective tubeweld, a critical area of a tube weld being that region where a tube anda membrane bar join; and, (c) simultaneously exposing each saidradiation receiver to said radiation source thereby creating an image ofsaid perimeter portion of each said tube weld.
 2. A radiographic methodof inspecting tube welds as set forth in claim I further comprising thestep of creating an approximately actual size image of each said tubeweld.
 3. A radiographic method of inspecting tube welds a set forth inclaim 2 further comprising the step of using radiographic film andaffixing it to cover the far wall critical area of each said tube weld.4. A radiographic method of inspecting tube welds as set forth in claim3 further comprising the initial step of removing a segment of saidmembrane bar intermediate said adjacent tube welds, said removed segmentbeing sized to permit the installation of said radiation sourceintermediate said adjacent tube welds.
 5. A radiographic method ofinspecting tube welds as set forth in claim 4 further comprising thestep of securing said radiation receiver around at least half thecircumference of each said tube weld.
 6. A radiographic method ofinspecting tube welds in tubular membrane panel walls comprising thesteps of:(a) installing a radiation source intermediate an adjacent pairof tube welds in a tubular membrane panel wall; (b) securing a radiationreceiver around a perimeter portion of each said tube weld, the maximumdistance of said source to any region of each said radiation receiverbeing no more than approximately 1 to 11/2 diameters of its respectivetube weld; and, (c) simultaneously exposing each said radiation receiverto said radiation source thereby creating an image of said perimeterportion of each said tube weld.
 7. A radiographic method of inspectingtube welds as set forth in claim 6 further comprising the step ofcreating an approximately actual size image of each said tube weld.
 8. Aradiographic method of inspecting tube welds as set forth in claim 7further comprising the step of using radiographic film and affixing itto cover the far wall critical area of each said tube weld.
 9. Aradiographic method of inspecting tube welds as set forth in claim 8further comprising the initial step of removing a segment of saidmembrane bar intermediate said adjacent tube welds, said removed segmentbeing sized to permit the installation of said radiation sourceintermediate said adjacent tube welds.
 10. A radiographic method ofinspecting tube welds as set forth in claim 9 further comprising thestep of securing said radiation receiver around at least half thecircumference of each said tube weld.